Don't import `QtGui` when loading `QtCore` unless required

[StSav012]

To make up mightBeRichText, QtGui is being imported in the following lines:

qtpy/qtpy/QtCore.py

Lines 103 to 112 in b851e00

	# Missing QtGui utility functions on Qt
	if getattr(Qt, "mightBeRichText", None) is None:
	try:
	from PySide2.QtGui import Qt as guiQt
	Qt.mightBeRichText = guiQt.mightBeRichText
	del guiQt
	except ImportError:
	# Fails with PySide2 5.12.0
	pass

qtpy/qtpy/QtCore.py

Lines 132 to 137 in b851e00

	# Missing QtGui utility functions on Qt
	if getattr(Qt, "mightBeRichText", None) is None:
	from PySide6.QtGui import Qt as guiQt
	Qt.mightBeRichText = guiQt.mightBeRichText
	del guiQt

I suppose the function is not something widely used, so the imports slow the module initiating down.

I'd implement the function this way (and similarly for PySide2):

    # Missing QtGui utility functions on Qt
    if not hasattr(Qt, "mightBeRichText"):
        def _mightBeRichText(*args, **kwargs):
            from PySide6.QtGui import Qt as guiQt

            return guiQt.mightBeRichText(*args, **kwargs)

        Qt.mightBeRichText = _mightBeRichText

Furthermore, this seems to bypass PYSIDE-2898.

But then, what about convertFromPlainText, which is also defined in QtGui.Qt?